In a number of applications, thin coatings of alloy materials are used on metal structures to protect them from the damaging effects of high temperature and/or corrosive or oxidizing environments. For example, in gas turbine engines many of the components must withstand high stress while enduring a corrosive gas stream whose temperature may be as high as 2500.degree. F. In such an environment, protective coatings on the metal components are essential to the satisfactory performance of the turbine. In many conventional turbines, the metal components are comprised of the class of materials which have become known as "superalloys", and the protective coating is chosen from one of the materials known as MCrAlY coatings, where M is selected from the group consisting of iron, nickel, cobalt and certain mixtures thereof. Such substrate and coating compositions are well known in the art and are described in, for example, U.S. Pat. No. 4,419,416.
Since the coatings involved play such a vital role in protecting the underlying metal structures, it follows that it is also important that the coating be properly applied to the metal substrate during manufacturing. Hence, a necessary part of the quality control program for manufacturing such parts is to determine the thickness of the protective coating layer on the finished articles. Furthermore, if it is determined that the coating is somehow defective or that the metal substrate must be reworked for some other reason, then the coating must be stripped off and then reapplied to the reworked part. Such protective coatings are typically removed by a chemical etching process that preferentially attacks the coating. However, the chemical etchants conventionally used will also attack the exposed substrate surface if the etching process is allowed to continue for too long a period of time. For coatings which have been heat treated to form a diffusion bond between the coating and the substrate, allowing the etching process to continue for too long results in intergranular attack by the etchant on the substrate surface. When the coating is reapplied to a substrate which has been damaged by this type of intergranular attack, the coating will contain cracks. Due to variances in the stripping rate associated with such parameters as the bath temperature, concentration of the etchant, thickness variations of the coating on the part, etc., it is often very difficult to sufficiently control the etching process so as to ensure removal of the coating while simultaneously preventing stripping of the underlying substrate.
Thus, both for manufacturing new metal components having protective coatings thereon and for reworking existing ones, it is necessary to be able to determine the presence and thickness of the coating. For purposes of checking quality control, one method for making this determination would be to simply section a sample of the finished article and directly measure the coating thickness. Of course, the destructive nature of this type of determination limits its utility to applications where "spot checking" the coating presence and thickness is sufficient. For checking the coating thickness of parts which will be used in production and for detecting whether the coating has been removed from parts which are being reworked, a non-destructive testing system is needed. The system employed should be capable of being used for relatively complex part geometries, such as those which are conventionally utilized in turbine buckets. Preferably, the thickness of the coating is measurable even when the coating thickness varies with location on the part. Furthermore, the testing system should be capable of detecting relatively small coating thicknesses, even for coating compositions which are very similar to the composition of the underlying substrate.
It is well known that if two dissimilar metals are joined together, a thermoelectric potential exists across the metal junction. If the junction is either heated or cooled, a different thermoelectric potential will appear across the junction, with the difference between the two thermoelectric potentials being proportional to the difference in temperatures between the ambient condition and the heated or cooled state. This principle has been employed for many years to measure temperatures with metal-junction thermocouples. Furthermore, for a given temperature, the magnitude of the thermoelectric potential across a metal junction depends upon the composition of the metals which form the junction. Thus, instruments have been built for sorting materials by determining the thermoelectric potential across a junction formed between a metal of known composition and the material to be sorted, and then comparing the resulting thermoelectric potential to predetermined values associated with various junction compositions at the given temperature.
U.S.S.R. Pat. No. 110,354, filed on Mar. 26, 1957, discloses that a galvanic coating thickness can be determined by measuring the thermoelectromotive force in the base metal and its coating metal. The magnitude of the measured thermoelectromotive force is then compared with the values corresponding to various coating thicknesses at the temperature involved. According to that patent, the thermoelectromotive force is determined by connecting an electrical return to the base and contacting the coating material with a heated probe, and then measuring the magnitude of the resulting thermoelectromotive force by connecting a galvanometer between the probe and the electrical return. In the other method disclosed, the electrical return is replaced by a second heated probe. The subject patent further indicates that, for cases where contact with the base is not possible, the measurement may be made by using two probes of different materials, with each probe contacting the coating surface.
The present inventor has found that these prior art techniques and procedures do not produce satisfactory results when applied to the problem of determining the thickness of the protective coating which is applied to the metal components of a gas turbine engine. For such applications, the thermoelectric potential available from the metal junction formed at the coating/substrate interface is quite small because of the similarities in composition between the coating and the substrate. Hence, the apparatus utilized to measure the magnitude of the thermoelectric potential must be capable of discriminating the voltage signal from the background noise. The measurement obtained should also be relatively immune from the effects of thermal drift of the measuring circuit. In order to minimize the number of parameters that might vary from one reading to another, for varying coating thicknesses, and thereby increase the probability that the change in thermoelectric potential being measured is due to a corresponding change in coating thickness, it is preferable that a single heated probe is employed in the inventive apparatus rather than the two heated probes that are disclosed in the aforementioned U.S.S.R. Pat. No. 110,354. For similar reasons of simplicity it is also preferable that the two probes employed in the apparatus be constructed of the same material, even for measuring the coating thickness of an article for which contact between the second probe and the substrate is not possible. Moreover, for the types of probes which are conventionally employed to heat a metal surface, oxidation and/or corrosion of the probe surface, as well as mechanical wear of the probe tip caused by repeatedly touching the probe to the metal surface, can cause variations in the amount of heat transferred from the probe to the surface and thereby affect the thermoelectric potential readings obtained. Additionally, the present inventor has determined that, for common probe materials such as copper, small amounts of the probe material deposited on the coating surface by scuffing of the probe against the coating can form eutectics on the coating surface when subjected to the elevated temperatures encountered in the operating environment of a gas turbine. Due to these and other problems, the generally recognized and long-standing need in the metal plating/stripping industry for a system that provides non-destructive determination of the presence and thickness of a protective coating on a metal substrate has not been met by any of the concepts known in the prior art.
Accordingly, it is an object of the present invention to provide a method and apparatus for non-destructively determining the thickness of a coating on a metal component.
It is also an object of the present invention to provide a coating measurement system that is effective even for thin coatings and complex substrate geometries.
It is a further object of the present invention to provide a system for determining coating thicknesses in which the results obtained are not sensitive to the passage of time.
It is still another object of the present invention to provide an apparatus which is especially useful for determining the thickness of the protective coating on parts which are used in conventional gas turbines.